I have a problem building an aggregate function. Here's my problem: I have a table like this id action day isSum difference 1 ping 2012-01-01 1 500 (this is the sum of the differences from last year) 2 ping 2012-01-01 0 -2 3 ping 2012-01-02 0 1 4 ping 2012-01-03 0 -4 5 ping 2012-01-04 0 …